Rice can be categorized as non-glutinous rice and glutinous rice according to transparency, and non-glutinous rice is transparent and glutinous rice is milky and non-transparent. Starch, which is a major component of rice, consists of amylose and amylopectin which are polysaccharides. Glutinous rice mainly consists of amylopectin, and non-glutinous rice consists of 15 to 20% of amylose and 80 to 85% amylopectin. Accordingly, when glutinous rice is cooked or processed to produce rice cake, the glutinous rice is much stickier than the non-glutinous rice, is not hardened even when stored for a long period of time, and retains its springiness and softness for a long period of time. Due to such a big difference in starch tissue between non-glutinous rice and glutinous rice, they are used for different purposes and have different qualities of processed food even when the same process method is used.
Retrogradation of starch refers to local formation of a crystal structure by combination of starch molecules through a hydrogen bond when gelatinized starch is left at room temperature for a long period of time. Due to the formation of hydrogen bond in starch, water present in starch is released out and thus, a product is hardened. Factors that affect retrogradation of starch include various additives and physical and chemical conditions, such as the kind of starch, a ratio of amylose to amylopectin in starch, a storage temperature, a pH, and water content. Many efforts have been made to suppress retrogradation of starch by using trehalose, a surfactant, an emulsifier, an oligosaccharide, or an amylase for saccharifing fresh starch. However, most conventional retrogradation suppression methods include the use of an additive, a preservative, etc. Accordingly, there is still a need to develop a rice cake that has good texture without use of food additives, and has a long storage time enabling the rice cake to be not hardened at room temperature for a long time period.